Reliability-Based Economic Design Optimization of Spread Foundations

This technical note develops a design approach that integrates economic design optimization with reliability-based methodologies to rationally account for geotechnical-related uncertainties. The geotechnical related uncertainties are addressed using a reliability-based approach in the assessment of ultimate limit state (ULS) and serviceability limit state (SLS) requirements. This design approach is illustrated using an example of spread foundation under drained uplift loading. Comparison of the economically optimized design with conventional designs shows that cost of the economically optimized design is lower than that of other feasible designs, and increasing foundation depth is a relatively effective way to increase uplift capacity. Impacts of geotechnical property uncertainties on foundation construction costs are quantified, and the results form a basis of a quantitative cost-benefit analysis of reducing geotechnical property uncertainties. Operative horizontal stress coefficient (K) is shown to be the key parameter that significantly affects the design of a spread foundation under drained uplift, and therefore, deserves attention in site investigation. For a typical allowable uplift displacement y(a)=25 mm, the spread foundation design is dictated by the ULS requirement, and the effect of y(a), or SLS requirement, is negligible.